Waste gas heat exchanger

ABSTRACT

The invention relates to a waste gas heat exchanger comprising at least one bank of tubes ( 2, 3 ) through which waste gas can flow, the tube ends ( 4, 5 ) being respectively connected to a tube bottom ( 6 ) in a material fit, and a housing envelope ( 7 ) which surrounds the bank of tubes and through which a coolant can flow, one of the two tube bottoms being connected to the housing envelope (fixed bearing) in a fixed manner. According to the invention, the other tube bottom ( 6 ) is embodied as a movable bearing and forms a sliding seat ( 13 ) with the housing envelope ( 7 ), said sliding seat being in communication with the coolant side ( 8 ) on one side thereof and with the outer side (atmosphere) on the other side thereof.

[0001] The invention relates to an exhaust gas heat exchanger according to the preamble of patent claim 1. Such an exhaust gas heat exchanger has been disclosed by DE-A 199 07 163 of the applicant.

[0002] In this known type of construction, the tube ends of a tube bank are mounted in a tube sheet in corresponding openings and are welded to the tube sheet. The tube bank together with the two tube sheets is arranged in a casing, the tube sheets being welded at the circumference to the casing shell. The tubes are therefore firmly connected to the casing via the tube sheet, a factor which may lead to thermal stresses under certain conditions. Hot exhaust gas flows through the tubes on the inside and colder coolant flows through them on the outside, the coolant also flowing around the inside of the casing. In particular in the case of greater tube lengths of such an exhaust gas heat exchanger, e.g. for commercial vehicles, the different expansions of tubes and casing shell may lead to stresses which are no longer inadmissible, which may result, for example, in destruction of the tube sheet connection.

[0003] It has therefore already been proposed in EP-A 0 930 429 to design the one tube sheet of a tube bank as a fixed bearing and the other tube sheet as a movable bearing in the form of a sliding seat. In this case, the tube sheet is mounted and sealed off at the circumference in a sliding seat of the casing. The exhaust gas tubes can therefore expand independently of one another relative to the casing. However, this solution has the disadvantage that the coolant side is connected to the exhaust gas side via the sliding seat, so that, if the seal fails, coolant can pass into the exhaust gas flow, a factor which may lead to undesirable consequences during exhaust gas recirculation, e.g. engine damage.

[0004] It is therefore the object of the invention to improve an exhaust gas heat exchanger of the type mentioned at the beginning to the effect that no thermal stresses occur on the one hand and no mixing of coolant and exhaust gas flow occurs on the other hand.

[0005] This object is achieved by the features of patent claim 1. Due to this design of the movable bearing as a sliding seat, the exhaust gas side is reliably separated from the coolant side. The casing shell slides with its outside (or also with its inside) in a (or on a) casing seat fastened to the tube sheet; in the event of any leakage, the coolant escapes outward, i.e. mixing with the exhaust gas flow is ruled out. At significant temperature differences between the exhaust gas tubes and the casing shell, the tubes will expand to a greater extent than the casing: in this case, the casing shell “stands still”, whereas the tube sheet “grows” beyond the margin of the casing shell. In this respect, the entire exhaust gas heat exchanger becomes longer, although this expansion can be absorbed by the exhaust gas lines on the inlet and outlet sides of the exhaust gas heat exchanger by a correspondingly pliant design or a resilient mounting.

[0006] Advantageous configurations of the invention follow from the subclaims. Accordingly, advantageous sealing of the casing shell relative to the casing seat, that is to say on the coolant side, is obtained by an O-ring, which permits sliding of the casing shell in the casing seat.

[0007] According to an advantageous development of the invention, the O-ring can be accommodated in a ring arranged in a fixed position with respect to the casing, as a result of which deformations of the thin-walled casing shell are avoided and an improved sealing effect is achieved.

[0008] In a further advantageous configuration of the invention, a connection flange, adjoining which is a diffuser or an exhaust gas pipe, is provided on the other side of the tube sheet, that is to say toward the exhaust gas side. This flanged connection results in a favorable means of fitting the exhaust gas heat exchanger in the exhaust gas system.

[0009] Exemplary embodiments are shown in the drawing and are described in more detail below. In the drawing:

[0010]FIG. 1 shows a first embodiment of a sliding seat, and

[0011]FIG. 2 shows a second embodiment of a sliding seat.

[0012]FIG. 1 shows a cutaway section 1 of an exhaust gas heat exchanger (not completely shown) as used in exhaust gas recirculation, in particular in commercial vehicles having diesel engines. In this illustration, only one end of the tube bank of the exhaust gas heat exchanger is shown, this end being designed as a movable bearing. The other end (not shown) of the tube bank is designed in a known manner as a fixed bearing, i.e. the tube sheet is firmly connected to the casing shell, e.g. by laser welding. Of the entire tube bank, only two exhaust gas tubes 2 and 3 are shown here, through which exhaust gas flows in the interior. The tube ends 4 and 5 are mounted in a tube sheet 6 and are integrally connected to the latter, preferably by laser welding. Brazing would likewise be possible. The entire tube bank is surrounded by a casing shell, of which only a cutaway section 7 is shown here. Coolant flows in the intermediate spaces 8 between casing shell and tube 3 and in the intermediate spaces 9 between the individual tubes 2 and 3, this coolant passing through an inlet (not shown) into the casing and leaving the latter via an outlet (not shown) . The coolant is extracted from the coolant circuit of the internal combustion engine of the motor vehicle. The tube sheet 6, which is designed as approximately 2 mm thick metal sheet, projects outwards beyond the circumference of the casing shell 7 and therefore has an annular flange 10 which is provided with holes 11 distributed over the periphery. Located on the left-hand side in the drawing, that is to say on the side facing the casing shell 7, is an annular sealing flange 12 which is designed as a casing seat and has a sliding surface 13 corresponding to the circumference and the contour of the casing shell 7, that is to say a circular sliding surface 13 for example. Between the sliding surface 13 and the tube sheet 6 or the annular flange 10, an annular groove 14 is incorporated in the flange 12 and accommodates an O-ring 15 for sealing the casing shell 7 relative to the flange 12. The outer surface of the casing shell 7 and the sliding surface 13 therefore form a sliding seat, i.e. a movable bearing. The end face 16 of the casing shell 7 is set back from the tube sheet 6 by the size x, x corresponding to the relative expansion between the tubes 2, 3 and the casing shell 7. A connection flange 17, via which a diffuser (not shown) or an exhaust gas pipe (likewise not shown) can be connected, is located on that side of the annular flange 10 which is on the right in the drawing, that is to say toward the exhaust gas side. Both flanges 12 and 17 have holes 18 and 19 in alignment with the hole 11, so that all the flange parts 12, 10 and 17 can be firmly connected to one another via a screwed connection (not shown). A gasket 20 is inserted between the annular flange 10 and the connection flange 17 for sealing off the exhaust gas.

[0013] The thermal expansion at different temperatures of the exhaust gas tubes and of the casing shell takes place in the following manner: the casing shell 7, which is connected, for example, to the engine block (not shown) via holders (not shown), virtually stands still. If the exhaust gas tubes 2, 3 expand to a greater extent than the casing shell 7, the tube sheet 6 “grows” beyond the end face 16 of the casing shell by the size x. In the process, it pushes the two flanges 12 and 17 with it to the right, so that the casing seat with the sliding surface 13 and the O-ring 15 slide on the casing shell 7. At the same time, however, the sealing of the coolant space 8 to the outside is ensured. Since the flange 17, which is followed by the further conduction (not shown) of the exhaust gases, likewise “grows” at the same time, i.e. it is displaced by the size of the expansion, a correspondingly “pliant” design of the exhaust gas system is required. However, this is not a technical problem with regard to the relatively small expansions. Since the tube sheet 6 emerges outward into the annular flange 10 and the latter is tightly connected to the connection flange 17 via the gasket 20, the exhaust gas space is hermetically sealed off from the coolant space, so that mixing of exhaust gas and coolant is ruled out.

[0014]FIG. 2 shows a further exemplary embodiment for the design of the sliding seat, i.e. of the movable bearing. Here, a tube sheet 30 has an angled annular flange 31 which is offset parallel to the tube sheet plane and extends in the radial direction beyond a casing shell 32. As in the exemplary embodiment according to FIG. 1, exhaust gas tubes 33 and 34 are welded in place in the tube sheet 30 and therefore form a tight and firm tube sheet connection. The tubes 33, 34 are part of a tube bank (not completely shown) which is completely surrounded by the casing shell 32, through which coolant flows. Fastened, e.g. brazed or welded, to the end face 35 of the casing shell is an annular part 36 which with regard to its wall thickness is dimensioned to be larger than the casing shell 32. Toward its outside, the annular part 36 has an annular groove 37, into which an O-ring 38 is inserted. Fastened to the annular flange 31 is a sealing flange 39, which has a sliding surface 40 on its side facing the casing shell 32, this sliding surface 40 together with an outer surface 41 on the annular part 36 forming a sliding seat. In this exemplary embodiment, in contrast to the exemplary embodiment according to FIG. 1, the annular groove having the O-ring is arranged on the casing side; this has the advantage that the relatively thin-walled casing shell 32 is reinforced in the end-face region and thus no deformations which result from the pressure load on the coolant side and which could lead to leakage at the O-ring seal occur in this region.

[0015] In both aforesaid exemplary embodiments according to FIG. 1 and FIG. 2, the sliding seat or the sealing and sliding surface is arranged on the outside of the casing shell. In principle, however, it is also possible to provide a corresponding sliding seat on the inside of the casing shell, in which case part of the sliding seat would in turn be connected to the tube sheet. 

1. An exhaust gas heat exchanger having at least one tube bank, through which exhaust gas can flow and the tube ends of which are each integrally connected to a tube sheet, and having a casing shell which surrounds the tube bank and through which a coolant can flow, and that one of the two tube sheets is firmly connected to the casing shell (fixed bearing), characterized in that the other tube sheet (6, 30) is designed as a movable bearing and forms together with the casing shell (7, 32) a sliding seat (13, 40, 41) which is connected to the coolant side (8) on the one hand and to the outside (atmosphere) on the other hand.
 2. The exhaust gas heat exchanger as claimed in claim 1, characterized in that the sliding seat (14, 40, 41) is arranged on the outside of the casing shell (7, 32).
 3. The exhaust gas heat exchanger as claimed in claim 1, characterized in that the sliding seat (13, 40, 41) has an annular groove (14, 37) with an O-ring (15, 38).
 4. The exhaust gas heat exchanger as claimed in claim 1, characterized in that the tube sheet (6, 30) has an annular flange (10, 31) which projects beyond the casing shell (7, 32) and carries the sliding seat (13, 40, 41).
 5. The exhaust gas heat exchanger as claimed in claim 4, characterized in that the sliding seat (13, 40) is formed by a sealing flange (12, 39) which is connected to the annular flange (10, 31).
 6. The exhaust gas heat exchanger as claimed in claim 4, characterized in that an annular part (36) is fastened to the end face of the casing shell (32), this annular part (36) accommodating the O-ring (38) in an annular groove (37) and, together with a sealing flange (39) fastened to the tube sheet (30, 31), forming the sliding seat (40, 41).
 7. The exhaust gas heat exchanger as claimed in claim 1, characterized in that the annular flange (10) is connected via a gasket (20) to a connection flange (17) for the exhaust gas line.
 8. The exhaust gas heat exchanger as claimed in claim 1, characterized in that the flange parts (12, 10, 17) form a flanged connection. 